1. Field of the Invention
The present invention relates to a signal detection technique, such as modulation scheme identification, and more particularly, to a modulation scheme estimation apparatus and method for automatically detecting modulation scheme of a modulation signal.
2. Description of the Related Art
As a conventional technique of automatically detecting modulation scheme of a digital modulation signal, a linear modulation identification method has been proposed, which includes two processes to be executed successively, i.e., the process of identifying, using amplitude distribution characteristic extraction, the orthogonal amplitude modulation schemes and phase modulation schemes of M-Quadrature amplitude modulation (M-QAM), Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), π/4-QPSK and M-Phase Shift Keying (M-PSK), and the process of identifying BPSK, QPSK, π/4-QPSK and M-PSK, using angle distribution extraction (see, for example, JP-A 2001-86171 (KOKAI)).
Further, a linear-modulation identification method, in which Cross Margenau-Hill Distribution (CMHD) that is known in the field of quantum mechanics is applied to modulation scheme estimation, has been proposed (see, for example, H. Ketterer et al., “Classification of modulation modes using time-frequency methods,” ICASSP '99. Proceedings, Vol. 5, pp. 2471-2474. March 1999).
However, in the above-mentioned prior art documents, after orthogonal amplitude modulation and phase modulation are discriminated and isolated from each other during the estimation of modulation scheme, it is necessary to again estimate each modulation scheme of orthogonal amplitude modulation and phase modulation. If there is an error in the initial isolation of orthogonal amplitude modulation and phase modulation from each other, the error not only adversely affects estimation of each modulation scheme, but also reduces the fairness (in probabilistic meaning) of discrimination. As a result, discrimination errors may well be increased. (See, for example, JP-A 2001-86171 (KOKAI).) Furthermore, modulation scheme estimation using CMHD exhibits a problem when realizing real-time processing, since it requires a greater number of operations (see, for example, H. Ketterer et al., “Classification of modulation modes using time-frequency methods,” ICASSP '99. Proceedings, Vol. 5, pp. 2471-2474. March 1999).